make rebuild-ft-billing-for-day remove unused rows

if the billing data was incorrect and needs to be rebuilt, we should
remove old rows. Previously we were only upserting new rows, but old
no-longer-relevant rows were staying in the database. This commit
makes the flask command remove *all* rows for that service and day,
before inserting the rows from the original notification data after.

This commit doesn't change the existing nightly task, nor does it
change the upsert that happens upon viewing the usage page. In normal
usage, there should never be a case where the number of billable units
for a rate decreases. It should only ever increase, thus, never need to
be deleted

app/dao/fact_billing_dao.py

@@ -125,6 +125,18 @@ def fetch_monthly_billing_for_year(service_id, year):
     return yearly_data
 
 
+def delete_billing_data_for_service_for_day(process_day, service_id):
+    """
+    Delete all ft_billing data for a given service on a given bst_date
+
+    Returns how many rows were deleted
+    """
+    return FactBilling.query.filter(
+        FactBilling.bst_date == process_day,
+        FactBilling.service_id == service_id
+    ).delete()
+
+
 def fetch_billing_data_for_day(process_day, service_id=None):
     start_date = convert_bst_to_utc(datetime.combine(process_day, time.min))
     end_date = convert_bst_to_utc(datetime.combine(process_day + timedelta(days=1), time.min))